1. Field of the Invention
This invention relates to a television sound receiver and more particularly to a television sound receiver which improves the tone quality of the television sound signal.
2. Description of the Prior Art
As television multiplexed sound broadcasting becomes more widely used, the tone quality of the television sound signal, which has the been more or less overlooked uptil now, becomes more worthy of notice.
Generally, in receiving and demodulating television sound signals intercarrier and split-carrier sound demodulating systems are typically employed. These systems will be described hereinafter.
FIG. 1 is a system diagram showing an intercarrier sound demodulating system in which a signal received at an antenna 1 is supplied to a tuner 2 from which an intermediate frequency (IF) signal containing a video carrier component with a frequency f.sub.p of, for example, 58.75 megahertz (MHz) and a sound carrier component with a frequency f.sub.s of, for example, 54.25 MHz are produced. This IF signal is supplied to a video intermediate frequency amplifier 3 as well as to a filter 4 which is provided to extract only the video and sound carrier components. An output signal from filter 4 is supplied through an amplifier 5 to a sound detector 6. Sound detector 6 produces a frequency modulated (FM) sound signal as a beat frequency signal of 4.5 MHz and is equivalent to the difference between the video and sound carrier frequencies. An output signal of sound detector 6 is applied to a frequency discriminator 7, frequency demodulated therein and an output therefrom is supplied to a multiplexed sound signal detector 8. Decoder 8 produces normal monaural sound signals as output signals S.sub.A and S.sub.B when the transmitted television sound is not multiplexed and a left-channel sound signal (one language sound signal) as output signal S.sub.A and a right-channel sound signal (another language sound signal) as output signal S.sub.B when the transmitted television sound is multiplexed such as in stereo sound. One part of the signal derived from sound detector 6 is fed back to amplifier 5 so as to provide an automatic gain control (AGC) thereto.
FIG. 2 is a system diagram showing an outline of the split-carrier sound demodulating system wherein an output signal of tuner 2 is supplied to a surface acoustic wave filter 9 having a band pass characteristic such that only the sound carrier component with frequency f.sub.s 54.25 MHz is produced. This sound carrier component is applied through an amplifier 10 to a mixer 11. Additionally a local oscillator 12 is provided which supplies an oscillating signal of 64.95 MHz to mixer 11 and thereby permits the oscillating and sound carrier signals to be multiplexed with each other. Accordingly, mixer 11 produces an output signal equal to a frequency difference between the sound carrier component and the oscillating signal of 10.7 MHz. The mixer output signal is supplied via a ceramic filter 13 to and demodulated at a frequency discriminator 14 which produces an output signal that is supplied to multiplexed sound signal decoder 8 and thereby produces the same output signals S.sub.A and S.sub.B as shown in FIG. 1.
The output signal of frequency discriminator 14 is supplied to local oscillator 12, forming an automatic frequency control (AFC) loop, to control the local oscillating frequency of local oscillator 12.
Since the aforementioned intercarrier sound demodulating system utilizes a frequency component of 4.5 MHz, which is the difference between the video and sound carrier component, the intercarrier sound demodulating system has an inherent drawback in that the video carrier component is apt to be mixed with the sound channel resulting in interference such as buzz noise.
In monaural sound broadcasting when the transmitted sound signal is not multiplexed, a high band component of the sound signal is deemphasized by a deemphasis circuit of a sound FM signal wave detector resulting in a somehwat acceptable level of buzz noise. In contrast thereto, when the sound signal is multiplexed, as in the present invention, the tone quality of the television sound is significantly and undesirably influenced by the buzz noise interference since it is necessary to extract a sub-channel sound signal before the entire sound signal is deemphasized by the deemphasis circuit.
During operation of the split-carrier sound demodulating system, the sound and video carrier components are separated from each other. Thus the sound carrier component is never influenced by the video signal component and hence the sound signal reproduced by the receiver has a good tone quality.
There are, however, drawbacks in using a split-carrier sound demodulating system. For example, since the sound and video carriers are adapted to be processed independently of each other in the split-carrier sound demodulating system, when the local tuner oscillating frequency is deviated and if the AFC is unlocked, due to, for example, changing the tuning frequency of tuner 2, the television sound signal can be lost even though the picture is still being reproduced on the screen of the receiver. Of course, the loss of a sound signal may not be readily noticed by viewers in general if the television receiver comprises only a video monitor having no sound reproducing apparatus or the demodulating decoder of the multiplexed sound signal is physically separated and located far from the television screen. But, when both the television receiver and demodulating decoder are integrally constructed and if only the picture and not the sound signal is reproduced, a viewer may not understand what has happened and may become puzzled or confused over the loss of the sound signal.
In contrast thereto, with respect to the intercarrier sound demodulating system, since the frequency difference between the video and sound carriers is utilized, the picture and sound signals of the television receiver will always be synchronized with each other such that the sound signal can never be lost while the picture signal is reproduced.
Additionally, although in receiving a very high frequency (VHF) signal, no buzz or buzz beat noise occurs in the split-carrier sound demodulating system such that the tone quality of a reproduced sound signal is satisfactory, when receiving an ultra high frequency (UHF) signal, the sound signal tone quality can become unacceptable. In fact, when receiving UHF signals, the split-carrier sound demodulating system can produce an inferior tone quality to that of the intercarrier sound demodulating system.
For example, in Japan, the local oscillating frequency in the tuner is 58.75 MHz higher than a received signal frequency. If a received signal, i.e., an input signal, has a low level, the local oscillator of the tuner can stably or positively oscillate at a predetermined local oscillating frequency. If the input signal level becomes high, however, a disturbing wave is superimposed upon a frequency determining element such as a variable capacitor of the tuner local oscillator which results in the local oscillating frequency being forcibly shifted. That is, due to the influence of the high level of the video carrier, a so-called "pull-in" operation of the tuner local oscillator occurs. It should be noted that the AGC is applied to the tuner in order to secure an acceptable S/N (Sound to Noise) ratio of the picture for both VHF and UHF input signals whenever the input signal level is 65 dB.mu. or more. Thus when an input signal is at or above 65 dBu, the AGC will suppress and thereby significantly minimize the aforementioned pull-in operation of the tuner local oscillator. However, when the input signal is below 65 dBu, the AGC is inoperative. Thus for input signal levels just below 65 dBu, the pull-in operation can occur without being suppressed by the AGC. In such instances, if the input signal is at an ultra high rather than very high frequency, the frequency determining element, such as the variable capacitor, will be more sensitive to the high level of the input signal resulting in much greater frequency fluctuation of the oscillator output signal and creating appreciable buzz noise interference. For example, the sensitivity of a variable capacitor at an ultra high as compared to a very high frequency can be raised by a factor of four. Therefore, in case of the UHF, if the pull-in operation, as set forth above, occurs at an input signal level of about 50 to 60 dB.mu. which, can not be suppressed by the AGC, the tuner local oscillating frequency will change in accordance with the input signal level. In a television receiver, since the video carrier is amplitude modulated (AM), the tuner local oscillating frequency fluctuates in response to the level of the video carrier. Thus in the split-carrier sound demodulating system, when fluctuations in the tuner local oscillating frequency occur due to fluctuations, in a UHF video carrier, the sound signal fluctuates and becomes buzz noise.
The degree of tone quality deterioration, due to buzz noise, becomes rapidly worsens when the AGC is ineffective in the case of a UHF signal. After the buzz noise is detected and if the input signal level is further raised by about 5 dB, the tone quality deterioration becomes even more severe as compared with the tone quality of the intercarrier sound demodulating system.
To prevent such deterioration, the AGC of the UHF tuner must be arranged such that the AGC becomes effective when the input signal level is still 50 to 60 dB.mu.. However, by lowering the threshold level which activates the AGC, the S/N ratio of the picture will deteriorate, as set forth above, and becomes unsuitable for practice. To prevent such S/N deterioration, a buffer amplifier inserted between the local oscillator of the UHF tuner and the mixer has been considered. However, it is quite difficult to design a buffer amplifier which is capable of covering the entire UHF range. Furthermore even if such a design were possible, it would be quite difficult to produce such an amplifier at an attractive manufacturing cost.
An additional problem arises in the split-carrier sound demodulating system due to amplitude to phase modulation (AM-PM) coversion prior to transmission of a signal. More specifically, since the video carrier as well as the sound carrier are modulated for phase in satellite multiple relaying broadcasting, in the intercarrier sound demodulating system, the phase modulated amount is cancelled whereby no buzz noise occurs. But, in the split-carrier sound demodulating system, since the sound carrier component modulated for phase is separated from the video carrier component reproduced as it is demodulated, the phase modulated amount is not eliminated so that buzz noise occurs.
Thus, as described above, neither the split-carrier nor the intercarrier sound demodulating system is always suitable in improving the tone quality of the television sound signal and each system has various defects.
In order to take advantage of the best aspects from both systems a television receiver, as shown in FIG. 3, which includes a multiplexed sound signal decoder can be used.
In FIG. 3, a television sound receiver is provided which includes both intercarrier and split-carrier sound demodulating circuits and is designed to select the demodulated output signal produced from either circuit.
The intercarrier sound demodulating circuit is supplied by an output signal derived from tuner 2 which applied to video intermediate frequency amplifier 3. An output signal from the video intermediate frequency amplifier 3 is delivered to a video detector circuit (not shown) as well as sound demodulating circuit 6. The frequency modulated sound signal derived from sound demodulating circuit 6 is supplied to and frequency-demodulated by frequency discriminator 7, which provides an output signal that is delivered to a signal switching circuit 16.
The split-carrier sound demodulating circuit also supplied by the output signal derived from tuner 2 which is applied to surface acoustic filter 9. An output signal from filter 9, which contains the sound carrier component only, is applied and converted to a sound carrier signal having a frequency of 10.7 MHz by a frequency converter or converting circuit 15. An output signal from frequency converter 15 is supplied to and demodulated at a frequency discriminator or discriminating circuit 14. The demodulated output therefrom is supplied to signal switching circuit 16 which selectively switches from the demodulated output signal supplied by frequency discriminator 7 to the demodulated output signal supplied by frequency discriminator 14 such that the output signal from frequency discriminator 14 is delivered to multiplexed sound signal decoder 8.
A control circuit for switchably controlling signal switching circuit 16 includes a low pass filter 17 and comparator 18. The output signal from the frequency discriminator 14 is supplied to low pass filter 17 which produces an S-shaped output signal (refer to FIG. 4) and which is delivered to comparator or comparing circuit 18.
When the receiving condition is not acceptable and the frequency of the local oscillator (not shown), which is included in tuner 2, is altered by, for example, .+-.250 kHz or more due to, for example, fine turning of tuner 2, comparator 18 produces, for example, a detecting high level output signal "1". The detected output signal "1" is supplied to signal switching circuit 16 as the control signal through an OR circuit 19 so that signal switching circuit 16 is switched to supply the demodulated output delivered from frequency discriminator 7 in the intercarrier sound demodulating circuit to multiplexed sound signal decoder 8.
When the receiving condition is satisfactory, however, comparator 18 does not produce the detecting output "1" so that signal switching circuit 16 is switched to the demodulated output derived from frequency discriminator 14 of the split-carrier sound demodulating circuit which is delivered to multiplexed sound signal decoder 8.
Accordingly, since comparator 18 does not produce the detecting output "1" under normally satisfactory receiving conditions, the demodulated output signal provided by the split-carrier sound demodulating circuit is supplied to multiplexed sound signal decoder 8 and thereby reproduces a television sound signal with good tone quality and no buzz noise interference.
When the frequency error of the local oscillator included in tuner 2 reaches or becomes greater than a predetermined value and when fine tuning or other tuning is performed at tuner 2, such that the AGC is unlocked, the comparator 18 produces detecting output "1" resulting in the output signal produced by the intercarrier sound demodulating circuit delivered to multiplexed sound signal decoder 8. Thus, the receiver, as shown in FIG. 3, will never allow the sound signal to be lost while the picture is reproduced on the picture screen.
Additionally, a forced change-over switch (not shown) can be included in the receiver to force the signal switching circuit 16 to switch from the split-carrier sound demodulating circuit to the intercarrier sound demodulating circuit. For example, when a UHF signal is received by antenna 1 and when the buzz noise is remarkable in the reproduced or outputted sound, through a terminal 20 a forced intercarrier switching control signal having the same high level as the detecting output "1" derived from comparator 18, is supplied through a terminal 20 to signal switching circuit 16 via OR circuit 19 and thereby forces the receiver to switch from the split-carrier sound demodulating circuit to the intercarrier sound demodulating circuit. Therefore, the previously mentioned drawbacks caused by employing only the split-carrier sound demodulating circuit are avoided.
The television sound receiver as described above, with reference to FIG. 3, however, has the following inherent problem: Due to the characteristic of frequency discriminator 14, an output voltage E produced from low pass filter 17 provides an S-shaped characteristic as illustrated in FIG. 4. The output voltage E of low pass filter 17 varies in accordance with the change of an input signal frequency f applied to filter 17. More specifically, as shown in FIG. 4, character f.sub.0 represents a tuning frequency of frequency discriminator 14 wherein when f=f.sub.0, E=0. When the f.sub.1 .ltoreq.f.ltoreq.f.sub.2, E and f form a linear relationship and when f&lt;f.sub.1 and f&lt;f.sub.2, E=0. Output voltage E is equal to E.sub.1 and E.sub.2 when f is equal to f.sub.1 and f.sub.2, respectively. Also, f.sub.1 ' and f.sub.2 ' are respectively selected such that f.sub.1 &lt;f.sub.1 '&lt;f.sub.0, f.sub.0 &lt;f.sub.2 '&lt;f.sub.2 and f.sub.2 '-f.sub.0 =f.sub.0 -f.sub.1 '. The corresponding values of E when f=f.sub.1 ' and f=f.sub.2 ' are E=E.sub.1 ' and E=E.sub.2 ', respectively. Furthermore, when the local oscillator included within tuner 2, as set forth above, is deviated by .+-.250 kHz or more, tuning frequency is f is equal to f.sub.2 ' and f.sub.1 ', respectively.
In comparator 18, voltages E.sub.1 ' and E.sub.2 ' are predetermined reference voltages such that when E satisfies the conditions E.sub.1 '.ltoreq.E.ltoreq.E.sub.2 ', in other words, when f is within a range of f.sub.1 '.ltoreq.f.ltoreq.f.sub.2 ' shown by reference letter a in FIG. 4, the detecting signal becomes "0" as mentioned above. Thus, signal switching circuit 16 is switched to supply the demodulated output signal derived from the split-carrier sound demodulating circuit to multiplexed sound decoder 8.
Additionally, when E is less than E.sub.1 ' or greater than E.sub.2 ', that is, when E&lt;E.sub.1 ' or E&gt;E.sub.2 ', tuning frequency f is less than f.sub.1 ' or greater than f.sub.2 ', that is, f&lt;f.sub.1 ' or f&gt;f.sub.2 ', respectively, as shown by letter b in FIG. 4. In particularly, for those values of frequency f where f.sub.1 .ltoreq.f&lt;f&lt;f.sub.1 ' or f.sub.2 '&lt;f.ltoreq.f.sub.2 the detecting signal becomes "1" as described above. Thus signal switching circuit 16 is switched to supply the demodulated output signal from the intercarrier sound demodulating circuit to multiplexed sound signal decoder 8.
However, when large deviations occur in the local oscillating frequency of the local oscillator included within tuner 2, tuning frequency f contained in the input signal applied to frequency discriminator 14 becomes greater than f.sub.2 or less than f.sub.1, that is, f&gt;f.sub.2 and f&lt;f.sub.1 and E=0. Therefore, the value of E (E=0) is the same as when f=f.sub.0. Thus, the detecting signal becomes "0" instead of "1" and thereby causes misoperation of signal switching circuit 16 resulting in the demodulated output signal delivered from the split-carrier sound demodulating circuit rather than from the intercarrier sound demodulating circuit being supplied to multiplexed sound decoder 8.